1. Field of the Invention
The present invention relates to an information recording device such as for example an FDD, wherein the information is accessed by rotating a disk-shaped recording medium.
2. Description of the Related Art
An FDD (floppy disk drive) is employed for the writing or reading of information to or from disk-shaped information recording media called floppy disk that is used in word processors or personal computers. The spindle motor that is employed to effect rotary drive of the floppy disk in an FDD needs to have a wide torque range and high rotational accuracy is demanded, so, conventionally, a DC brushless motor was usually employed. A DC motor is available at low cost and can provide high torque but requires a brush and so suffers from problems regarding reliability and is therefore not used much. In contrast, a DC brushless motor suffers from no problems at all with respect to reliability, but needs to be provided with a position detector and is therefore unavoidably expensive.
Also, whether a DC motor or DC brushless motor is employed, rotary speed feedback control needs to be provided by adding a speed detector. This means that motor control is by analogue means and the circuit layout becomes large, resulting in a system that cannot meet current demands for the lowering of costs.
On the other hand, with the progress in digital electronics technologies in recent years, in low-cost two-phase claw pole type stepping motors, techniques have become established for achieving rotation with high rotational accuracy as a synchronous motor by a micro step drive system, and a digital control circuit can be constructed without needing to add position detectors or a speed detector in the interior of the motor. This leads to the advantage that the system as a whole can be constructed at low cost. The adoption of such a system is therefore being studied. However, since it is a synchronous motor, it has the drawback that, in the load torque to motor current characteristic, motor current does not increase and decrease in proportion to the load (as it does with a DC motor), so it cannot cover a wide range of torque (0-80 g.cndot.cm) such as is required for an FDD. As a measure for this drawback the current is determined so as to match maximum load, resulting in that large current is supplied even at low load. This has the disadvantages of being uneconomic in regard to power-saving of the device and, in addition, of leading to large vibration/noise due to the current being set at high value.